This invention relates to atomization and application of fluids, such as paints, to a surface and to apparatus which minimizes loss of such fluids into the atmosphere.
Airless spraying equipment operate using pressures of 1800 to 2500 psig. Through the use of hydraulic pressure, the fluid is conveyed to a spraying apparatus where it is forced through a small orifice. The high pressure by which it is propelled is responsible for "bounceback" whereby the fluid literally bounces back into the atmosphere of the work place. It contaminates the environment, the worker, and the equipment.
The most prominent method of spraying a liquid or a powder is to use a high pressure gas, such as air, to entrain and carry the liquid or powder to a substrate or target. The high pressure gas explodes into the atmosphere creating a turbulence and finely particulates the solids. This turbulence dispenses the particles over a large area producing a deleterious fog or mist of toxic fumes and harmful solids. The danger to the worker, to the environment, and to cost containment is obvious.
Conventional pneumatic spraying apparatus use high pressure, low volume compressor air at 50 to 60 psig and 4 or 5 cfm in concert with an air regulator to atomize fluids. Spraying with such an apparatus produces a wasteful cloud of fluid and air commonly referred to as "overspray". Overspray is created by the explosive expansion of the mixture of solids, liquids, and gas at the nozzle. Overspray contains an aerosol of fluid drops and solid particles including drops of less than 10 microns in diameter. Solvents in the fluid being sprayed are referred to as volatile organic compounds (VOC). VOC entrained in overspray rapidly evaporates. The VOC become part of the atmosphere and present a hazard to the environment and to the operator.
Low pressure high volume (LPHV) spraying apparatus uses air at a low pressure (up to 10 psig) and high volume (up to 22 cfm). LPHV spraying reduces the incidence of bounceback because the fluid sprayed contacts the surface at a relatively low velocity. LPHV spraying reduces the incidence of overspray because the explosive expansion atomization of fluid which produces the aerosol is minimized when low pressure air is used.
In conventional usage, "overspray" is used as a generic term which includes bounceback and overspray as described above, and is sometimes called errant spray. This generic usage will be used here.
U.S. Pat. No. 4,850,809, incorporated herein by reference, discloses an apparatus for LPHV spraying.
The measurement for comparing methods of atomization is called transfer efficiency (T.E.). T.E. is expressed as a percentage of the solid substances that become part of a substrate or arrive at the intended target. Conventional pneumatic spraying has a T.E. of 25%; airless spraying has a T.E. of 40%; and LPHV spraying has a T.E. of 75%.
The Environmental Protection Agency has expressed special concern about the hazards associated with airborne particles of a diameter of 10 microns or less (PM.sub.10). That Agency has established regulations controlling PM.sub.10 concentrations in outdoor applications, such as shipbuilding, bridges, towers, and architectural coatings. The production of PM.sub.10 is virtually uncontrollable when conventional spraying methods are used.
The production of VOC is often regulated in terms of tons VOC/day emitted per site. A typical spray booth is ventilated by a flow of air at 150 ft.sup.3 /minute per ft.sup.2 surface being painted. The contaminated air is then treated to remove the VOC and PM.sub.10, often by incineration, a very expensive process.
Two trends have emerged from efforts to protect the environment from solvents and aerosols resulting from overspray. In order to prevent bounceback spraying pressures are limited to 10 psig in some locations. In order to reduce solvent entry into the atmosphere, fluid formulations containing as much as 70% solids are often used.
The present invention captures and resprays the overspray, thereby increasing the transfer efficiency of the process and reducing the air pollution due to VOC and PM.sub.10 associated with spraying.
The respray system reduces the total amount of pigment needed by capturing and reapplying pigment which otherwise would be lost in overspray. In addition, the recycle of VOC prevents stripping of solvent from pigment during spraying, which results in undesirable premature drying of the sprayed paint. This will result in a net reduction of volume of solvents which must be used for a given amount of pigment.
U.S. Pat. No. 2,438,471 discloses a curtain of air introduced around the spray nozzle which traps the rebounding portions of the coating mixture and forces it against the surface being coated. The air curtain is emitted through a series of holes in an annular air chamber extending entirely around the spray nozzle.
U.S. Pat. No. 1,897,173 discloses a cap like spray nozzle in which a central stream of liquid is surrounded by an annular air port. The liquid stream is modified by two opposed supplemental air ports which shape the emitted spray into a fairly sharply defined ellipsoid cross-section.
U.S. Pat. No. 2,101,922 discloses an apparatus for spraying melted paraffin onto porous surfaces. The stream of paraffin is surrounded by a sheath of heated air. One venturi arrangement is used to atomize and propel the paraffin is a stream of air while a second concentric venturi is used to provide the sheath of heated, low-pressure air.
U.S. Pat. No. 4,664,060 discloses a paint spray booth for painting motor vehicles. A venturi is used to intermix paint particle-bearing air and water in order to scrub the paint particles from the air.
There is a continuing need for spraying apparatus which will allow spraying of atomizable liquids at a high transfer efficiency and minimal air pollution.